1. Field of the Invention
The present invention relates to the lithographic processing of semiconductor wafers, and more particularly to suppressing alignment mark reflection.
2. Discussion of the Prior Art
To align a film mask with a semiconductor wafer surface, an alignment mark on the film mask can be aligned with an etched mark on the wafer. Alignment marks in lithographic processes can be difficult to use because of the level of contrast between the mark and the surrounding area. Alignment marks can be used for lithographic processing of semiconductor wafers where light is projected through the film mask onto the surface of a wafer coated with a photosensitive material. A substantial contributor to reduced contrast between the alignment mark and surrounding structures is light reflection.
Reflection can be reduced through the use of thin-film coatings, wherein the thin-film coatings are anti-reflective (AR) films. AR films can be employed on the surfaces of solar cells, light-sensitive detectors, displays, lenses, and the like, to suppress undesirable Fresnel reflections.
Another method for reducing reflection is to provide an incident surface with a periodically structured array, such that the periodicity is smaller than the wavelength of the incident light. Such a surface behaves like an effective medium, in which the effective index of refraction changes continuously from that of the air to that of the material. The result is that incident light is reflected at every depth of the structure, such that the reflected light is out of phase, thereby generating destructive interference.
Surface texturing of silicon surfaces to enhance absorption is known for solar cell surfaces. For solar cells, surface textures comprising arrays of microscopic pyramids have been used on to increase absorption of incident light. These structures can be formed by a process known as interferometric lithography, also known as optical interference lithography.
However, no known system or method current exists in the field of semiconductor lithography for reducing the reflectivity of an alignment mark using surface texturing. Therefore, a need exists for a system and method of suppressing reflection of an alignment mark and thereby increasing contrast.